I wanted to get back to this with a practical example, but the weather has not been cooperating to say the least. Still, these extremely poor pictures show the issues to expect. Wide field shows the area in front of my house, with uncorrected vignetting. The white spot is the area we are aiming at. 1st zoom level is taken with the Canon 300 2.8 + TC 1.4 (420mm) 2nd zoom is the TMB 805/115 at prime focus. The two flat fields (that's the plus of bad weather, you can take flat fields ;-)) show that the amount of vignetting is, expectedly, much higher with the TMB than it is with the Canon Zoom. This being said, compared to the wide angle shot, which can be corrected in lightroom, it remains quite acceptable. The problem is of course that you'll have to make a model of the vignetting yourself if you care. The second image shows a roof on the edge of the field of view and, imho, distortion is kept well under control in the TMB (no need for a field flattener).
Now, about the constraints... The Canon + TC was handheld with IS on (I made sure exposure was quick enough so vibration wouldn't impact the image too much) The TMB was on a 26kg mount, quite unpractical to catch a bird in flight, and I had to take 5 shots around teh focus point to be sure I would hit something decent (can't trust my eyes too much).
With even more constraints, I can bet better results with afocal images because the camera (using a macro lens in my case) is doing the autofocus on the image that gets out of the eyepiece, but the conditions weren't worth the trial